Abstract
Abstract Alfvénic waves have gained renewed interest since the existence of ubiquitous propagating kink waves were discovered in the corona. It has long been suggested that Alfvénic waves play an important role in coronal heating and the acceleration of the solar wind. To this effect, it is imperative to understand the mechanisms that enable their energy to be transferred to the plasma. Mode conversion via resonant absorption is believed to be one of the main mechanisms for kink wave damping and it is considered to play a key role in the process of energy transfer. This study examines the damping of propagating kink waves in quiescent coronal loops using the Coronal Multi-channel Polarimeter. A coherence-based method is used to track the Doppler velocity signal of the waves, which enables us to investigate the spatial evolution of velocity perturbations. The power ratio of outward to inward propagating waves is used to estimate the associated damping lengths and quality factors. To enable accurate estimates of these quantities, we provide the first derivation of a likelihood function suitable for fitting models to the ratio of two power spectra obtained from discrete Fourier transforms. Maximum likelihood estimation is used to fit an exponential damping model to the observed variation in power ratio as a function of frequency. We confirm earlier indications that propagating kink waves are undergoing frequency-dependent damping. Additionally, we find that the rate of damping decreases, or equivalently the damping length increases, for longer coronal loops that reach higher in the corona.
Highlights
Magnetohydrodynamic (MHD) waves are a common phenomena in the solar corona and a plethora of different wave modes have been observed in recent years as instrumentation has become increasingly sophisticated, offering higher spatial and temporal resolutions
Following Verth et al (2010), we focus our attention on half of a coronal loop and assume the kink waves at the coronal footpoint of the segment have a certain power spectrum, Pout(f ), where the subscript out refers to the fact they are outwardly propagating along this segment
In particular we are keen to examine whether the wave paths determined from following the Alfvenic fluctuations are situated in the POS, which has been the implicit assumption in previous analyses (Tomczyk & McIntosh 2009; Verth et al 2010)
Summary
Magnetohydrodynamic (MHD) waves are a common phenomena in the solar corona and a plethora of different wave modes have been observed in recent years as instrumentation has become increasingly sophisticated, offering higher spatial and temporal resolutions. Of the different MHD wave modes, Alfvenic waves are considered one of the main candidates for explaining the raised temperature in the corona. 2002; Goossens et al 2002; Aschwanden et al 2003). In the presence of structuring in the direction perpendicular to the magnetic field (i.e. the loop plasma is considered denser than the ambient plasma), transverse motions generate an intrinsic coupling between the kink (transverse) and Alfven (azimuthal, m = 1) modes. Aschwanden et al 2003; Goossens et al 2006; Antolin et al 2015) The coupling takes place in a dissipative layer at the loop boundary, located at the resonant point where the kink frequency, which lies between the internal and external Alfven frequencies, matches the local Alfven wave frequency (e.g. Aschwanden et al 2003; Goossens et al 2006; Antolin et al 2015)
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